Geothermal Power as Baseload Power in Small Island Contexts
Small islands face various energy challenges, including low purchasing power for traditional diesel generation, vulnerability to fluctuating oil prices, and microgrids vulnerable to weather events without connection to other islands or power sources for back-up power. Governments and utilities on many geologically active islands are exploring geothermal energy as a potential renewable energy source. The constancy of steam flow from geothermal resources to be used as base load is an attractive complement to intermittent wind and solar renewable energy resources. However, using geothermal plants as the majority of installed electric capacity on a microgrid presents challenges that must be considered. Notably, geothermal units are normally base loaded, lack dispatchability, and provide minimal spinning reserves. These units cannot be easily turned on and off due to steam-field operations and steam pressures. As loads typically decrease overnight, there are challenges with over-generation and providing adequate spinning reserves, when generation remains high but demand is typically low.
DNV GL has recently completed grid integration and power simulation studies in small island contexts, where the impacts of the integration of potential geothermal plants with existing diesel generation and intermittent renewable energy projects was assessed. In collaboration with utilities, several resource mix scenarios were developed representing possible geothermal, diesel, wind, and solar plant sizes and combinations.
DNV GL evaluated both the economic and technical impacts of the integration of the potential geothermal resources on the generation, transmission and distribution systems owned and operated by island utilities. These studies evaluate impacts on steady state operation (power flow, voltage profile, and thermal capacity), short circuit performance (changes in available fault current following integration of the new resources), and transient and dynamic performance (the ability of the system to maintain synchronism following a major system disturbance).
Generally, DNV GL finds that when geothermal plants make up a significant portion of the island’s total installed generation capacity, they have the capacity to significantly modify the volt-ampere reactive (VAR) flow, creating over- and under-voltages. Installation of additional capacitance near load centers can help mitigate these effects. Additionally, geothermal plants made up of a few large units may be subject to system instability when a single large generating unit goes offline (including large diesel or geothermal units). Implementation of storage may help mitigate but not fully solve these issues. Use of smaller geothermal units (e.g., 500 kW versus 5 MW) that can be individually turned off or down may also provide greater flexibility and ultimately stability. Finally, improvement in geothermal technology that allows units to have greater turn-down capability, or the ability to be valved-off completely, may also help resolve stability during periods of low load. This would likely increase the initial plant and infrastructure costs but could alleviate the need for storage and/or mitigate the stability issues. Determining the precise geothermal net size and configuration to maintain system stability is certainly specific to each island’s needs, but it is likely that increased granularity of the geothermal units (e.g., 500 kW geothermal units instead of 5 MW units) would improve the stability performance of the system.
In parallel, DNV GL has completed power simulation studies to evaluate the incremental impacts of geothermal resources in conjunction with renewable energy resources such as wind and solar on power supply costs for the islands’ utilities. Historically, small electric utilities have designed the generating resource mix to comprise mostly base load diesel generators and a few fast-start peaking generators. These existing units are typically older model units that tend to have high minimum generator operation levels due to the thermal heat rate curves and associated efficiencies. These units also tend to have increased start-up times. With electric utilities globally wanting to reduce the burning of oil and diesel fuel and increase renewable technology penetrations, these older units tend to create potential operating issues with stability and reliability due to the lack of spinning reserves and load following capabilities.
Spinning reserves are critical in ensuring grid stability and reliability. As noted, geothermal units typically only provide base load power, lack immediate dispatchability, and provide limited spinning reserves to support the reserves provided by the diesel units. As found in interconnection studies, the situation is exacerbated at night during low load levels when there is potential for over-generation from geothermal and potential intermittent resources coupled with a shortage of spinning reserves.
Variable generating renewable energy plants cannot provide adequate spinning reserves, since the power generation varies minute-by-minute depending on the environmental conditions (e.g., wind speed and sun irradiance). Electric utilities, therefore, tend to require increased spinning reserves to support varying renewable resources. Over-generation mainly occurs at night and weekends when customer loads are typically lower, but when renewable resources are still generating and some diesel generators are kept on line to provide spinning reserves and stability. Unless the renewable energy resources are curtailed, the electric utility may find itself operating the system without adequate reserves or operating the diesel generators at lower levels outside of their optimal efficiencies (i.e., to allow for more spinning reserves), both of which have negative consequences: Operating the system without adequate reserves can create stability issues during a generator or transmission line outage which can lead to cascading power outages; and operating diesel units outside of their economic efficiencies can increase fuel consumption/costs due to higher thermal heat rates when operating at lower capacities.
These potential problems may be mitigated with the installation of energy storage, increasing geothermal plant turn-down capability, replacing older diesel generators with new, more efficient units, or increased use of load shedding schemes. DNV GL expects to discover additional mitigation measures as geothermal technology improves, and as we complete future geothermal feasibility and grid integration studies.